Although there are a number of different types of thermal cut-off switches, many of them are overly complex and utilize more parts than are commercially feasible for many applications. The construction of a simple, inexpensive, but effective thermal cut-off device using a minimum number of parts, therefore, continues to be a challenge. One attempt at providing a simplified thermal cut-off switch design utilizing a minimum number of parts is shown in Japanese Utility Model Laid-Open Publication No. 15922/72, which was filed under the name of Murata Manufacturing Company Limited. In this type of device, a hollow structure having a normally elongated oblong shape with a continuous circumference is connected directly to the insulated lead.
When the pellet is inserted into the device and the oblong contact is forced against the pellet, it is bowed outwardly until it contacts the outer metallic housing of the device. The Murata device, however, can be difficult to manufacture due to the small size of the parts and because of the necessity of either providing either a hole in the contact through which the head of the insulated lead may pass, or of some other means of permanently securing the hollow contact to the head of the insulated lead which will not be affected by high temperatures. The contact of the Murata device engages the thermal pellet at essentially a single point, thereby subjecting the pellet to a high stress which tends to destroy the integrity of the pellet of organic chemical material and to reduce the reliability of the device because of premature opening of the switch.
Another design which utilizes a bowed hollow contact member with a continuous circumference or perimeter is shown in U.S. Pat. No. 4,167,724, issued Sept. 11, 1979, in the name of James R. McCaughna. The McCaughna device differs from the Japanese Murata device in that the hollow contact structure of this switch has a general rectangular shape and it is not permanently secured to the head of the insulated lead. Instead, the insulator through which the insulated lead passes has a reduced diameter portion which is encircled by a coiled spring which tends to force the contact member toward the thermally-sensitive pellet. The McCaughna thermal cut-off switch, has a flat contact area which engages the thermal pellet; and thus, there is a good force distribution on the thermal pellet in the McCaughna switch.
The present invention is described by reference to a thermal switch in which a McCaughna-type of electrical contact is employed. However, the McCaughna device contemplated the use of non-conductive thermal sensing pellets made of organic chemical material. Such pellets are ideal for certain temperature ranges. The organic chemical material does impose temperature range limitation upon the switch, however, and these materials are relatively expensive. In order to provide a switch with a reduced cost and an extended temperature range, the switch of the present invention employs a pellet of electrically-conductive solder material, the composition of which depends upon the temperature range desired. For example, conventional tin-lead solder mixtures of silver-solder mixtures may be employed. The design features of the present invention are directed to making the use of such materials possible in thermal sensing switches.